IRU1237SC datasheet(5/7 Pages) IRF

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IRU1237SC

Rev. 1.2

07/14/03

www.irf.com

APPLICATION INFORMATION

Introduction

The IRU1237SC regulator is 7-pin terminal device and

contains three integrated, fixed, linear regulators. The

dual inputs provide a low dropout voltage for V33 and V26

by biasing the base current to power NPN transistors.

The IRU1237SC is designed to meet the fast current

transient needs as well as an accurate initial voltage,

thus reducing the overall system cost with the need for

fewer number of output capacitors.

Thermal Protection

The IRU1237SC provides thermal protection for all three

outputs. All outputs will be disabled for any over-tem-

perature condition. When one of the outputs exceeds

the thermal limit (typically 150

8C), the IRU1237SC shuts

down all three outputs simultaneously. The outputs will

be re-enabled when the temperature drops below the

thermal limit.

Current Limit Protection

The IRU1237SC provides over current protection when

one the outputs’ current exceeds the current limit level.

Stability

The IRU1237SC doesn’t require input and output capaci-

tors for stability, however to improve the transient re-

sponse and guarantee stability, it is recommended that

a 0.1

mF (minimum) ceramic for input and output capaci-

tors be used.

Transient Response and PSRR

The input and output capacitors are critical in order to

ensure good transient response and PSSR. The most

important aspects of this are capacitor selection, place-

ment, and trace routing. Place each capacitor as close

as physically possible to it’s corresponding regulator pin.

Use wide traces for low inductance path. Couple directly

to the ground and power planes as possible. The use of

low ESR capacitors is crucial to achieving good results.

An input capacitance of at least 0.1

mF is recommended.

An output capacitance of at least 0.1

mF with low ESR is

recommended for good PSSR at high frequencies. Ce-

ramic capacitors are a good choice for low ESR. Larger

capacitance and lower ESR will improve both PSSR and

transient response.

VIN1=5.25V

VIN2=12V

IOUT1=0.5A

IOUT2=0.5A

IOUT3=0.1A

VOUT1=2.6V

VOUT2=3.3V

VOUT3=8V

RTH(JA)=30W/

TA = 50

DT = PD3RTH(JA) = 2.7W330 = 818C

TJ = TA+

DT = 1318C

PD = (VIN1-VOUT1)

3 IOUT1+(VIN1-VOUT2)3 IOUT2+(VIN2-

VOUT3)

3IOUT3

PD = (5.25-2.6)

30.5+(5.25-3.3)30.5+(12-8)30.1

PD = 2.7W

Thermal Design

The IRU1237SC incorporates an internal thermal shut-

down that protects the device when the junction tem-

perature exceeds the allowable maximum junction tem-

perature. Although this device can operate with junction

temperatures in the range of 150

8C, it is recommended

that the selected heat sink be chosen such that during

maximum continuous load operation the junction tem-

perature is kept below this number.

The following thermal design illustrates the method used

to calculate the maximum junction temperature of the

regulator.

Calculating the maximum power dissipation:

For Ultra Thin-PakTM we have:

Layout Consideration

The IRU1237SC, like many other high-speed regulators,

requires that the output capacitors be close to the de-

vice for stability. For power consideration, a ground plane

pad of approximately one-inch square on the compo-

nent side must be dedicated to device where all ground

pins are connected to dissipate the power. The copper

area under the package shall have vias to the internal

ground plane. The thermal ground plane shall extend out

from the regulator to open areas of the PCB. All open

areas shall be filled with copper to help radiate heat from

the PCB.

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